Method and device for testing electronic devices

ABSTRACT

The present invention provides a new electronic circuit board and a method of using such board to test electronic devices at elevated temperatures. The board comprises a steel base having a dielectric coating layer and a circuit formed on the layer. The circuit includes a connector region for attachment to an external electrical source and a mounting region for mounting sockets for supporting, powering and monitoring the electronic devices during elevated temperature testing. The board displays a leakage current of less than 10 μAmps at 350° C.

RELATED APPLICATIONS

[0001] This application claims the benefit of provisional applicationNo. 60/299,632 filed Jun. 20, 2001 and application Ser. No. 10/175,522filed Jun. 19, 2002, both of which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention concerns an electronic circuit board foruse in the high temperature testing of electronic devices and a methodfor conducting such tests. Such boards are commonly referred to asburn-in boards. The circuit board of the present invention comprises abase made of steel which is coated with a dielectric layer.

BACKGROUND OF THE INVENTION

[0003] The high temperature testing of electronic circuit devices iscommonly employed in the semiconductor or microelectronic devicemanufacturing industries. Such tests are utilized in the burn-in, TDDB(time dependent dielectric breakdown) and the electromigration testingof semiconductor devices or chips.

[0004] Burn-in testing comprises the application of thermal andelectrical stresses for the purposes of inducing the failure of marginalmicroelectronic devices having inherent defects resulting frommanufacturing aberrations which cause time and stress dependentfailures. TDDB testing concerns monitoring a device for diminishedelectric properties during heating. Electromigration in thin metal linescan cause chip failures with the formation of voids, or gaps, ininterconnects. The potential for electromigration failures is a moresignificant issue in standard aluminum metal interconnects as chipmakers attempt to reduce resistance using thinner wires. Copper metalinterconnects are more resistant to electromigration. However, migrationis still a concern and this concern has resulted in the need for testingprocedures at elevated temperatures. These various elevated temperaturetests are run both for the end-run qualification of chips and also forchip developmental purposes. These tests can also be run as a qualitycontrol test for incoming electronic devices or chips.

[0005] During all elevated temperature testing procedures, theelectronic devices are loaded into sockets which make temporaryelectrical contact with the device leads. The sockets are mounted onhigh temperature circuit boards with circuitry to provide the propervoltages and electric stimuli to the devices. These circuit boards,which are used in electromigration, TDDB and burn-in testing, arecommonly referred to as “burn-in boards.” Once the boards are filledwith devices, the boards are then loaded into a heating device, such asa convection oven, which in addition to supplying heat also provides anelectrical interconnect between the boards and the power supplies andsignal generators used to power the electronic devices during heating.During heating, the electrical characteristics of the electronic devicesare continually monitored, logged and made available for analysis. Theoven, power supplies and signal generators are commonly referred to asthe burn-in system.

[0006] In the prior art, many burn-in boards were constructed ofphenolic/epoxy materials. However, such boards present a major drawback.Specifically, such boards degrade quickly at elevated temperatures(e.g., temperatures in excess of about 180° C.). Ceramic substrates areemployed for tests run at elevated temperatures (e.g., 300° C.),however, ceramic substrates are costly and they must be limited in size,for they are somewhat fragile. Boards comprising porcelain enamel coatedsteel substrates have also been used in the prior art to produce burn-inboards. These boards have been sold by the ECA Electronics Company ofErie, Pa., under the trademark ELPOR. However, such boards display aleakage current of about 40 μAmps or more at 350° C. For the testing ofcurrent generation electronic devices, there is a need for boards thatdisplay a leakage current of less than about 10 μAmps. Accordingly,there is a need for a robust high-temperature burn-in board whichdisplays excellent electrical properties that can be produced at areasonable cost.

SUMMARY OF THE INVENTION

[0007] The present invention provides a new and improved electroniccircuit board or burn-in board for use in testing semiconductor chips orother electronic devices at elevated temperature and a method forconducting such tests. The burn-in board is extremely robust and it canbe produced at a reasonable cost. Such boards display a leakage currentof less than 10 μAmps at 350° C. Of course, leakage current is afunction of board geometry, circuit area and operating temperature.However, burn-in boards generally display a surface area of 70 or moresquare inches per side, with about 10%-20% of each side printed withcircuitry.

[0008] In one preferred embodiment the burn-in board comprises astainless steel base having a dielectric coating formed thereon. Thedielectric coating comprises multiple discrete layers of dielectricmaterial. Formed on the dielectric coating is an electrical circuit. Thecircuit includes a connector region for facilitating attachment to anexternal electrical source and a mounting region for mounting socketsfor supporting and supplying current to the electronic devices duringheating. The board displays a leakage current of less than 10 μAmps at350° C.

[0009] Among those benefits and improvements that have been disclosed,other objects and advantages of this invention will become apparent fromthe following description taken in conjunction with the accompanyingdrawings. The drawings constitute a part of this specification andinclude exemplary embodiments of the present invention and illustratevarious objects and features thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In the annexed drawings:

[0011]FIG. 1 is a top view of a burn-in board made in accordance withthe present invention having no sockets mounted thereon;

[0012]FIG. 2 is a top view of the burn-in board of FIG. 1 with socketsmounted thereon;

[0013]FIG. 3 is a schematic side view of the burn-in board of FIG. 2;

[0014]FIG. 4 is a schematic broken-away cross-sectional view of aportion of a burn-in board made in accordance with the presentinvention; and

[0015]FIG. 5 is a schematic broken-away cross-sectional view of aportion of another embodiment of a burn-in board made in accordance withthe present invention.

DETAILED DESCRIPTION

[0016] Referring to the drawings, and initially to FIGS. 1-3, there isillustrated a burn-in board or electronic circuit board 10 made inaccordance with the present invention. In all of the drawings, the samereference numerals are used to identify similar elements. Board 10comprises a metal base 12 coated along its top and bottom surfaces witha dielectric coating 14. The dielectric coating 14 in one embodimentcomprises two (2) discrete layers of porcelain enamel. As discussedbelow, the dielectric coating 14 may be formed in a variety of manners.

[0017] Board 10 includes along its top surface 16 an electrical circuit18 formed by use of a thick film conductor material. Circuit 18 includesa connector region 20 and a mounting region 22. The front end 24 ofconnector region 20 is where the board is plugged into an electricalreceptacle located in the heating device or oven wherein the board 10 isintended to be utilized. The mounting region 22 is the portion of theboard wherein the sockets 26 for supporting the electronic devices 27that are being tested are located. Sockets 26 may comprise one of avariety of commercially available sockets for supporting electronicdevices, such as, semiconductor chips, for testing. In addition toproviding mounting support, sockets 26, provide an electrical connectionbetween the circuit 18 and the devices 27.

[0018] Referring now to FIG. 4, there is schematically illustrated abroken-away cross-sectional view of a burn-in board 10 made inaccordance with the present invention. As shown, board 10 includes themetal base 12, and the dielectric coating or material layer 14. On topsurface 16 of dielectric layer 14 is the electronic circuit or circuittrace 18.

[0019] As illustrated, dielectric layer 14 is formed from two discretelayers 28 and 29 of dielectric material. Layers 28 and 29 are formedfrom different material systems, and thus they are nonhomogeneous. Baseor substrate 12 may comprise any one of a variety of high temperaturefiring metal materials. Examples of such materials include decarburizedsteel or stainless steel and it will be appreciated that the base may becoated on all sides or just one side depending upon circuitrequirements. Metal substrates coated with a dielectric layer ofelectronic grade porcelain enamel along all sides of the metal base arecommercially available under the trade designation ELPOR from the ECAElectronics Company located in Erie, Pa. Coated substrates arecommercially available from ECA Electronics made with either low carbonsteel or various grades of stainless steel. Applicants herebyincorporate by reference U.S. Pat. No. 6,195,881B1; Lim et al., U.S.Pat. No. 5,002,903; Ohmura et al., U.S. Pat. No. 4,361,654; Kaup et al.,U.S. Pat. No. 3,935,088; Moritsu et al., U.S. Pat. No. 4,172,733; VanderVliet, U.S. Pat. No. 4,085,021; Hang et al., U.S. Pat. No. 4,256,796;Andrus et al., U.S. Pat. No. 4,358,541; Chyung, U.S. Pat. No. 4,385,127;Gazo et al. U.S. Pat. No. 3,841,986 and Hughes U.S. Pat. No. 3,575,838for their teachings as to how to produce metal substrates coated with adielectric layer.

[0020] For certain test applications, a test board may be required thatprovides a maximum leakage current of 10 μAmps at 350° C. In suchapplications, a dielectric coating 14 formed from multiple dielectriclayers 28 and 29 of nonhomogeneous materials is required. Suchmultilayer boards may be formed by taking a porcelain enamel metalcoated substrate available from ECA Electronics Company under the tradedesignation ELPOR and coating it with a high performance electronicgrade porcelain enamel coating material available from the FerroCorporation of Cleveland, Ohio, under the trade designation QP-330. TheECA substrate with its enamel coating provides bottom dielectric layer29 and the QP-330 material provides top layer 28. QP-330 is applied wetto the ECA porcelain coated substrates and then fired at about 800° C.The QP-330 material may either be applied by dipping or screen printingto a thickness of about 0.002″ (after firing). One to three layers ofthe QP-330 material maybe applied successfully to the ECA porcelaincoated substrates. Applicants have found that the use of multiple layersof the QP-330 leads to improved breakdown current properties.

[0021] As shown in FIG. 4, an encapsulant layer 40 may be applied overthe conductive circuit 10 to add further protection. A suitableencapsulant layer may be formed using a glass encapsulant sold by theFerro Corporation of Cleveland, Ohio, under the trade designationA-3565. The glass encapsulant serves to prevent particulate migrationbetween individual circuit traces. The encapsulant may be applied, forexample, by screen printing directly on the thick film materials and thetop surface of the dielectric layer and then the entire board may befired at a temperature of about 625° C.

[0022] Test boards that display further improved electrical propertiesmay also be produced by forming a dielectric coating using multiplediscrete homogeneous layers of commercially available thick filmdielectric materials intended for use on metal substrates. Such boardsdisplay a maximum leakage current of 1 μAmp at 350° C. Examples of suchmaterials include a thick film material available from Electro-ScienceLaboratories, Inc. of King of Prussia, Pa., under the trade designation4924, thick film materials available from DuPont of Wilmington, Del.,under the trade designation 3500N and thick film materials availablefrom Heraeus of West Conshohocken, Pa., under the trade designationIP-222. These commercially available materials are preferably applied to430 stainless steel. These materials are intended for use in makingthick film heaters, but applicants have unexpectedly found them suitablefor use in the present invention.

[0023] The thick film dielectric materials are applied in multiplelayers upon the metal substrate and then they are fired at a temperatureof about 850° C. Prior to application of the dielectric materials thestainless steel surface is thoroughly cleaned. The layers are preferablyapplied by screen printing. However, other application techniques suchas spraying could be utilized. Each applied layer is dried prior toapplication of the subsequent layer. Referring to FIG. 5 there isillustrated a substrate 12 having multiple layers of dielectric material42, 44, 46 screen printed upon substrate 12. Burn-in boards made usingthree layers of the 4924 thick film material at a total thickness afterfiring per side of about 0.006″ display a maximum leakage current of 1μAmps at 350° C.

[0024] Electrical circuit 18 may be formed in a conventional mannerusing a suitable commercially available conductive thick film orcombination of commercially available thick films. The thick films areapplied to the top of the dielectric layer using conventionalapplication techniques, such as, for example, screen printing. Examplesof other possible, but generally less desirable application techniquesother than screen printing include, for example, spraying, dipping,spinning, brushing and application using a doctor blade. An example of asuitable thick film for use in the present invention is a gold thickfilm available from Electro-Science Laboratories, Inc. under the tradedesignation 8835. It will be appreciated that a multilayer circuitstructure may be formed by applying a dielectric thick film layer overthe conductive layer. After firing the dielectric layer may form as abase for printing an additional circuit using conductive thick filmmaterials.

[0025] The circuit boards 10 of the present invention allow for a methodof high temperature testing of electronic devices such as, semiconductorchips, at elevated temperatures, such as from about 350° C. to about500° C., or preferably from about 400° C. to about 500° C. Duringtesting, while at elevated temperature, controlled electric current orsignals are supplied to the devices under test, and the performanceproperties of such devices are tracked.

[0026] Although the invention has been shown and described with respectto preferred embodiments, it is obvious that equivalent alterations andmodifications will occur to others skilled in the art upon reading andunderstanding the specification. The present invention includes all suchequivalent alterations and modifications, and is limited only by thescope of the following claims.

What is claimed is:
 1. A method of conducting the high temperaturetesting of an electronic device comprising the steps of: (i) providingan electronic circuit board for supporting and supplying current to thedevice to be tested, said electronic circuit board comprising a steelbase coated with a dielectric material coating; (ii) providing electriccurrent to the electronic circuit board; and (iii) heating theelectronic circuit board to a temperature of from about 350° C. to about500° C.
 2. A method as set forth in claim 1 wherein said steel basecomprises stainless steel.
 3. A method as set forth in claim 1 whereinsaid electronic circuit board includes a connector region for attachmentto an external electrical source and a mounting region having socketsfor supporting said electronic device.
 4. A method as set forth in claim1 including the step (iv) of monitoring the electrical characteristicsof said electronic device during heating.
 5. A method as set forth inclaim 1 wherein said dielectric material coating comprises multiplediscrete layers of dielectric material, and said electronic circuitboard displays a leakage current of less than 10 μAmps at 350° C.
 6. Amethod as set forth in claim 1 wherein said electronic circuit boarddisplays a leakage current of less than 1 μAmps at 350° C.
 7. Anelectronic device made by the method of claim
 1. 8. A method as setforth in claim 1 wherein said dielectric material coating comprises afirst layer of porcelain enamel and a second layer of high-performanceporcelain enamel.
 9. A method as set forth in claim 1 wherein saiddielectric material coating comprises multiple discrete layers ofdielectric material.
 10. A method as set forth in claim 1 wherein saiddielectric material coating comprises two discrete layers of dielectricmaterial.
 11. A method as set forth in claim 8 wherein said first andsecond layers of enamel are nonhomogeneous.
 12. A method as set forth inclaim 1 wherein said dielectric material coating comprises at leastthree discrete layers of dielectric material.